Understanding Earthquakes: Types, Causes, and Impact on Buildings, Assignments of Geology

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Sept. 11, 2008

Today’s Topics:

More EQs

Reading: Abbott

Chapters 4 & 6

Homework due

Sept. 16

Geos 218: Geological Disasters & Society

Earthquake

Main Concepts:

Earthquakes Stress Faults Types of Faults Seismic Waves Seismographs Earthquake Location Earthquake Size Earthquake Hazards It’s not EQs, it’s buildings...

Iran earthquake triggers N-bomb test scare globally 11 Sep 2008, 0126 hrs IST,TIMES NEWS NETWORK & AGENCIES TEHRAN: At 6.1, the earthquake that rocked Iran's main oil port of Bandar Abbas, was rated "strong" but not seen as likely to cause major damage. But the shock took a toll of nerves in many world capitals as speculation that Iran had tested a nuclear bomb spread like wildfire. The hysteria soon died down, but not before the west wondered if its worst nightmare - of a nuclear bomb in the hands of a theocratic Islamic state - had come true after all. Can we tell the difference between an earthquake and a nuclear bomb? YES!

Building design Building design

Which building do you want to work in and Which building do you want to work in and

why?why?

Resistance to shear is critical.Resistance to shear is critical.

Building design - resistance to shear Building design - resistance to shear

Building design: Building design:

Avoid RESONANCE... Avoid RESONANCE...

Why is building damage selective?

Resonance Resonance

• If the period of the wave matches the period of the building, shaking is amplified and resonance results. Common cause of catastrophic failure of buildings
• Eliminate resonance:
  • Change height of building
  • Move weight to lower floors
  • Change shape of building
  • Change building materials
  • Change attachment of building to foundation structural engineering for seismic hazards

Elevated Highways: A Common Problem Elevated Highways: A Common Problem

Too much weight on concrete columns!Too much weight on concrete columns!

Freeway column failed in 1994 earthquake when brittle concrete cracked and steel rebar buckled. New columns have vertical steel bars wrapped by circular rebar,encased in concrete and confined by steel jackets encased in concrete.

Building design Building design

Resistance to shear is critical. Resistance to shear is critical.

• Horizontal shaking during earthquakes can do massive damage to buildings
• Acceleration
  • Measure in terms of acceleration due to gravity (g)
  • Weak buildings suffer damage from horizontal accelerations of more than 0.1 g
  • At isolated locations, horizontal acceleration can be as much as 1.8 g (Tarzana Hills in 1994 Northridge, California earthquake) Seismic intensity affected by Seismic intensity affected by different rock types -different rock types -

Amplitude of oscillation

Seismic waves passing into softer rock slow down and increase in amplitude to carry the same energy. When the SW period matches the period of the building - damage

Liquefaction Liquefaction – – wet sediments becomewet sediments become

““quicksandquicksand””

Anchorage, 1964

Earthquake prediction still eludes us Earthquake prediction still eludes us

• Which fault

will break?

• When will

it break?

• Size of

earthquake?

Earthquake prediction is basically still Earthquake prediction is basically still

just a probability gamejust a probability game

probabiliities probabiliities ofof EQsEQs b/4 2032 basedb/4 2032 based

on historic records, trenching, GPSon historic records, trenching, GPS

Seismic risk in U.S. Seismic risk in U.S.

Expected: California, Seattle, Salt Lake City Expected: California, Seattle, Salt Lake City

Unexpected: upstate NY, New Madrid, Charleston Unexpected: upstate NY, New Madrid, Charleston

ArizonaArizona

EarthquakesEarthquakes

- - -- - - Southwestern AZSouthwestern AZ Northwestern AZNorthwestern AZ Central MountainCentral Mountain regionregion 1887

ArizonaArizona

EarthquakesEarthquakes

1990-20011990-

- - -- - - SantaSanta^ RitasRitas^ & Madera Canyon Fan& Madera Canyon Fan

Physiographic provinces of western US Physiographic provinces of western US

Locate:Locate: Great ValleyGreat Valley Snake River PlainSnake River Plain Great Salt LakeGreat Salt Lake RockyRocky MtnsMtns The Basin andThe Basin and RangeRange Colorado PlateauColorado Plateau SaltonSalton SeaSea Rio Grande riftRio Grande rift TucsonTucson

• What is meant?
  • Comparable earthquake occurred in Alaska, 2002 on Denali fault
    • Over 140 seconds, ruptured 340 km in magnitude 7. earthquake with offsets up to 8.8 m
    • Significant directivity – triggered earthquake swarms up to 3,660 km to the southeast as far as Utah
    • Unpopulated area, minimal effects on people

The Big One

• 1857 Fort Tejon earthquake (m=7.9) on San Andreas fault
  • Over 130 seconds, ruptured 360 km with offsets up to 9.5 m
  • Relatively unpopulated area at that time
• San Bernardino section of San Andreas fault now has 3 million people

The Big One

San Andreas Fault Zone

• locked segments (sections ruptured during large eqs in 1906 and 1857)
• creeping segments with frequent small to moderate eqs (Parkfield) Different sections of the fault behave differently 1857 1906 1906 (M=7.8) 1857 (M=8.3)

Great 1906 San Francisco Eq. (M=7.8)

• 430 km of San Andreas fault between Cape Mendocino and San Juan Bautista shifted up to 6 m
• Intense ground shaking for about one minute in early morning hours
• Damage much worse in areas constructed on artificial fill rather than rock or consolidated sediment
• Many fires broke out and water lines were destroyed, making fire-fighting impossible – fires caused ten times more damage than shaking ‘locked’ section of fault

1989 Loma Prieta (world series) Eq. (M=6.9)

• Occurred at the fault bend near Santa Cruz Mountains
• Slip was not just horizontal Loma Prieta region was a seismic gap before eq.
• 67 killed
• 3,757 injured
• 12,000 homeless
• $6 billion lost Damage was mostly to poorly sited or designed structures REMEMBER: Earthquakes don’t kill, buildings do !!!

Compression where San Andreas bends Compression creates thrust faults that uplift mountains nearby Shortening near LA can generate M=6 eq. every six years Los Angeles San Andreas Fault 1971 San Fernando & 1994 Northridge Eqs. M=6. M=6.

Los Angeles San Andreas Fault 1994 Northridge Earthquake Pico blind thrust ruptured in magnitude 6. earthquake under Northridge (shaking fortunately directed away from Los Angeles)

• 61 people killed, 9, people injured, $20 billion in damages
• Similar to 1971 San Fernando Valley earthquake except shaking directed toward Los Angeles
• Outbreak of Valley Fever
• Landslides

U.S. Earthquakes 1899-

 Earthquakes occur throughout North America, not just in California  Occur in clusters, mostly in western North America but also in eastern North America and Hawaii Pacific NW - CascadesPacific NW - Cascades San Andreas FaultSan Andreas Fault Alaska & HawaiiAlaska & Hawaii Eastern California -Eastern California - Western NevadaWestern Nevada Yellowstone HotspotYellowstone Hotspot Eastern Basin & Range -Eastern Basin & Range - WastachWastach^ FrontFront Rocky MountainsRocky Mountains Rio Grande RiftRio Grande Rift

Western NorthWestern North

AmericaAmerica

---

The Basin & RangeThe Basin & Range

---

A region where theA region where the

continent is beingcontinent is being

pulled apartpulled apart^ – –

extendedextended

......

Distance from SLC to Distance from SLC to SFO has doubled inSFO has doubled in last 25 million years!last 25 million years!

Basin & Basin &

RangeRange

Basin & Range Basin & Range

Fault Models Fault Models

active EQ zones in the Basin & Range active EQ zones in the Basin & Range

intermountain seismic belt western Great Basin seismic trend

Fault Scarp from the 1915 Fault Scarp from the 1915 magmag 7.17.

earthquake in Pleasant Valley, Nevadaearthquake in Pleasant Valley, Nevada

Intermountain seismic belt Intermountain seismic belt

Western MTWestern MT

YellowstoneYellowstone

ID - WY borderID - WY border

SLC (Wasatch Front)SLC (Wasatch Front)

SW UtahSW Utah

North-trending curve 1,500 km long and 100-200 km wide separating Basin and Range from Colorado Plateau and Rocky Mountains The Wasatch Front The Wasatch Front

Boundary between Basin and Range and WasatchBoundary between Basin and Range and Wasatch

Range Range

Capable ofCapable of

large large EQsEQs

Last majorLast major

EQ ~1600 EQ ~

AD AD

1.5 million1.5 million

people at people at

fault fault

This exaggerated 3D view of the Wasatch Front, from Utah Valley to the Salt Lake Valley, gives you a sense of the abrupt nature and topography of fault- block mountains. (Digital maps produced by Eric H. Christiansen) Photo of exposure of the Wasatch Fault scarp. This is one of the best exposures anywhere along the 240 mile fault. Large earthquakes occur along the fault on average about every 350 years. This Provo segment of the fault last ruptured about the year 1400. (Photo by Bill Harris)

Intermountain Belt: Utah, Idaho,

Wyoming, Montana

Borah Peak, Idaho, 1983  Lost River fault ruptured in 7.3 earthquake  Borah Peak (Idaho’s highest point) 0.3 m higher, Thousand Springs Valley few meters lower  Ground shaking caused fountains of groundwater http://quake.wr.usgs.gov/research/deforma tion/modeling/papers/naturalhistory/natura lhistory

Intermountain Belt: Utah, Idaho,

Wyoming, Montana

Hebgen Lake, Montana, 1959  Two faults moved within five seconds of each other with magnitude 6.3 and 7.5 earthquakes  Created landslide that dammed canyon and formed Earthquake Lake  Dropped north end of Hebgen Lake 7-8 m, creating seiche that sloshed back and forth for almost 12 hours

difficult-to-explain earthquakesdifficult-to-explain earthquakes

1811-1812 New Madrid1811-1812 New Madrid

hugehuge ““feltfelt”” areaarea many events,many events, several very largeseveral very large lots of liquefactionlots of liquefaction paleoseismologypaleoseismology shows bigshows big^ EQsEQs^ herehere in 500, 900, 1300,in 500, 900, 1300, 1600...1600... USGS says 90%USGS says 90% probability ofprobability of magnitude 6-7 EQmagnitude 6-7 EQ there in next 50 yrsthere in next 50 yrs WHY THERE?WHY THERE?

FIG. 6.25FIG. 6.25

1886 Charleston, South Carolina Earthquake

• Massive earthquake in 1886 destroyed 90% of buildings
  • No surface faulting from earthquake
  • Below surface studies indicate at least five similar earthquakes in last 3,000 years – recurrence interval of about 600 years Learning Objectives

1. Know the different types of faults--strike-slip (both right- and left- lateral) and dip-slip (both reverse and normal)--and their motions.
2. Be able to define period, frequency, and amplitude of waveforms. Know the basic types of body and surface waves.
3. Know how to use P and S travel times to determine earthquake locations.
4. Know how Richter magnitude is determined, why its use is restricted now, and what other magnitude scales are preferred instead.
5. Know how Mercalli intensity is determined and why it is still used, despite the existence of several magnitude scales.
6. Understand elastic rebound and how earthquakes rupture.
7. Understand what contributes to strong ground shaking and how to make buildings more resistant to shear.
8. What types of earthquakes would you expect in North America and why?
9. What are the different types of possible earthquakes associated with the San Andreas fault?